1. Field of the Invention
The present invention relates to the production of semiconductor elements in clean rooms, and more particularly, to an apparatus for dealing with the various difficulties caused by static electrification. Such difficulties include breakdown and performance deterioration of semiconductor devices, surface contamination of products due to absorption of fine particles and operational faults of electronic instruments located in such clean rooms.
2. Description of the Related Art
As high integration, high speed calculation and energy conservation are promoted in semiconductor devices, the oxide insulation films of semiconductor elements have become thinner and the circuits and metal electrodes of such elements have been miniaturized, and thus, static discharge frequently causes pit formations in the elements and/or fusion or evaporation of metallic parts of the elements, leading to breakdown and performance deterioration of the semiconductor devices produced. For example, some MOS-FET and GaAs devices cannot withstand a voltage as low as 100 to 200 volts, and thus, it is frequently necessary to maintain the surface voltage of such semiconductor material elements at about 20 volts or lower. When semiconductor elements have completely broken down, the defect may be detected upon delivery examination. It is, however, very difficult to identify performance deterioration of such elements. In order to reduce static electricity related difficulties, the objective is to reduce to the extent possible the exposure of semiconductors to static electricity, that is, to prevent charged articles from approaching the semiconductor elements, and to neutralize all such charged articles. However, using prior art technology, it has not been possible to completely achieve such an objective. Examples of surface voltage measurements of various articles involved in the production of semiconductor devices include 5 kV for a wafer, 35 kV for a wafer carrier, 8 kV for an acrylic cover, 10 kV for a table surface, 30 kV for a storage cabinet, 10 kV for the technician's garments and 1.5 kV for a quartz palette.
Recent super clean room technology has made it possible to realize a flow of supplied clean air containing no particles having a diameter of 0.03 .mu.m or more. However, fine particles are inevitably generated from the presence of operators, robots and various manufacturing apparatus located in the clean rooms. Such internally generated particles may have a diameter in the range of 0.1 .mu.m to several tens of .mu.m, and when such particles are deposited on the wafers of LSI and VLSE devices having a minimum line distance which is as small as 1 .mu.m, the result is faulty products which reduces the production yield. It has been recently established that the deposition of fine particles on wafers is primarily attributed to electrostatic attraction and that the particular air flow patterns in the vicinity of the wafers is substantially unrelated to such deposition. Accordingly, prevention of such surface contamination of products due to the deposition of fine particles may only be achieved by the development of a technology for removing static electricity which does not directly relate to the technology for enhancing the cleanliness of clean rooms.
Furthermore, in the case wherein electronic equipment is located in the clean room, discharge currents created by the discharge of charged articles, for example charged human bodies and charged sheets of printer paper, may create static noise causing faults in the operation of the electronic equipment. To avoid such operational faults it is desired that the static electricity of charged articles existing in the clean room be eliminated.
To eliminate the above-discussed various difficulties caused by static electrification in the clean room, it is effective to neutralize the charged articles existing in the clean room. In cases where the charged articles are electrically conductive, neutralization can be carried out by simply grounding the charged articles so that static charges can be rapidly removed. However, from a practical standpoint it is impossible to ground all charged articles existing in the clean room, and in cases where the charged articles are insulators, they cannot be neutralized by grounding. As for wafers, although they are themselves conductive, they are transported and handled in cassette cases or palettes which are insulating. Accordingly, it is difficult to neutralize wafers by grounding. For these reasons, there have been proposed systems for removing static electricity which employ ionizers.
The underlying principle of such ionizer systems is as follows. In a clean room, air particles are removed by passing the air through filters in a flow direction, which is substantially one direction. An ionizer for ionizing air by corona discharge (ion generator) is disposed upstream the flow of clean air (normally in the vicinity of the air exhaling surfaces of the filters) to provide a flow of ionized air, which comes in contact with the charged articles to neutralize static electricity on the charged articles. Thus, positively and negatively charged articles are neutralized by negatively and positively ionized air, respectively.
Three general types of corona discharge ionizers are known--the pulsed DC type ionizers, the DC type ionizers and the AC type ionizers. In such ionizers, emitters are disposed in an air space and a high DC or AC voltage is applied to each emitter so that an electric filed of an intensity higher than the dielectric breakdown voltage of air is created in the vicinity of the emitter, thereby effecting corona discharge. The known types of air ionizers will now be described in some detail below.
Pulse DC type. As is diagrammatically shown in FIG. 19, direct currents having, for example, voltages of +13 kV to +20 kV and -13 kV to -20 kV, respectively, are alternately applied at a given time interval (e.g. from 1 to 11 seconds) to a pair of needle-like emitters (tungsten electrodes) 100a and 100b disposed spaced from each other by a predetermined distance (for example several tens of cm), whereby positive and negative air ions are alternately generated from each of the emitters 100a and 100b. The ions so generated are carried by air flow to a charged article 101 to neutralize static charges of opposite polarity on the article 101. An example of the DC pulse applied to the emitters is shown in FIG. 20.
DC type. As is diagrammatically shown in FIG. 21, a pair of insulator coated electrically conductive bars 102a and 102b respectively having a plurality of emitters 103a and 103b extending therefrom at 1 to 2 cm intervals, are disposed parallel to each other with a predetermined distance (for example several tens of cm) therebetween. A positive DC voltage (e.g. +12 to +30 kV) is applied to the emitters 103a of the bar 102a, while a negative DC voltage ((e g. from -12 to -30 kV) is applied to the emitters 103b of the bar 102b, thereby ionizing air.
AC type. An AC high voltage of a commercial frequency of 50/60 Hz is applied to needle-like emitters. As is diagrammatically shown in FIG. 22, a plurality of emitters 104 are arranged in a two dimensional expanse and connected to a high voltage AC source 105 via a frame work of conductive bars 106 having insulating coatings. For each emitter, a grounded grid 107 is disposed as an opposite conductor so that the grid 107 surrounds the discharge end of the emitter 104 with a space therebetween. When the high voltage AC is applied to emitter 104, there is formed an electric field between the emitter 104 and the grounded grid 107. This electric field inverts its polarity in accordance with the cycle of the applied AC, whereby positive and negative ions are generated from the emitter 104.
All such known types of ionizers pose various problems, as noted below, when they are employed to neutralize charged articles in a clean room.
Firstly, the emitters themselves contaminate the clean room. It is said that tungsten is the most preferred material for the emitter. When a high voltage is applied to the tungsten emitter to effect corona discharge, a great deal of fine particles (almost all of them having a diameter of 0.1 .mu.m or less) are sputtered from the discharge end of the emitter upon generation of positive ions, and are carried by the flow of the clean air to thereby contaminate the clean room. Furthermore, since the discharge end of the emitter is damaged by the sputtering, the emitter must frequently be replaced.
Secondly, when an ionizer is made to operate for a prolonged period of time in a clean room, white particulate dust (primarily comprised of SiO.sub.2) deposits and accumulates on the discharge end of the emitter to the extent that it may be visible. While the cause of such white particulate dust is believed to be attributed to the material constituting the filters, the deposition and accumulation of the particulate dust on the discharge end of the emitter poses a problem in that ion generation is reduced and contamination is increased due to scattering of the dust. Accordingly, the emitter must frequently be cleaned.
Thirdly, a plurality of emitters disposed on the ceiling of the clean room may increase the concentration of ozone in the clean room. Although the increased ozone concentration is not especially harmful to humans, ozone is reactive and undesirable in the production of semiconductor devices.
In addition to the above-discussed common problems, the individual types of known ionizers involve the following problems.
With DC type ionizers, in which some emitters (emitters 103a on the bar 102a in the example shown in FIG. 21) generate positive ions, while the other emitters (emitters 103b on the bar 102b in the example shown in FIG. 21) generate negative ions, and in which such ions are carried by the air flow, frequently there is an imbalance in the number of positive or negative ions which arrive at a charged article. The charged article often receives only ions having the same polarity as that of the static charge thereon. In this case the charged article is not neutralized. On the contrary, an uncharged article or slightly charged article may experience an increased charge as a result of the ions carried thereto. While such a phenomena is likely to occur in the case where the distance between the electrodes (the distance between the rods 102a and 102b in the example shown in FIG. 21) is fairly large, if the distance is made short to counter this problem, a new problem of sparking is posed.
With pulsed DC type ionizers in which the polarity of the ions is reversed at a predetermined interval, positive and negative ions are alternately supplied to the charged article. Accordingly, the condition in which an imbalance of positive or negative ions is continuously supplied to the charged article, as is the case with the DC type ionizers, is avoided. However, if the pulse period is short there is an increased possibility that the positive and negative ions will admix in the air flow and thus disappear before they reach the charged article. To the contrary, if the pulse period is long, although the possibility that the ions will disappear is decreased, large masses of positive and negative ions will alternately arrive at the charged article. It is reported by Blitshteyn et al. in Assessing The Effectiveness of Cleanroom Ionization Systems, Microcontamination, March 1985, pages 46-52, 76 that with pulsed DC type ionizers, the potential of a charged surface decays in a zigzag manner, for example, as shown in FIG. 23. According to this report, static electricity on a charged surface does not disappear, rather static loads of about +500 volts and about -500 volts alternately appear on the charged surface. Such a large surface potential may reduced the production yield since recent super LSI devices may be damaged even by a surface potential on the order of several tens of volts.
AC type ionizers suffer from an imbalance in the number of generated positive ions and the number of generated negative ions. Frequently, the number of positive ions generated is more than ten times the number of negative ions generated. Shown in FIG. 24 are measurement results reported by M. Suzuki et al. depicting the densities of the positive and negative ions generated by an AC type ionizer. See the Japanese language literature, Proceedings of The 6th. Annual Meeting for Study of Air Cleaning and Contamination Control, (1987) pages 269-276, and the corresponding English language literature, M. Suzuki et al., Effectiveness of Air Ionization Systems in Clean Rooms, 1988 Proceedings of The IES Annual Technical Meeting, Institute of Environmental Sciences, Mt. Prospect, Illinois, pages 405 to 412. As seen from FIG. 24, the density of negative ions is markedly lower than that of positive ions. The measurement as shown in FIG. 24 was made with an AC type ionizer installed in a space wherein clean air was caused to flow downwards in a vertical direction from horizontally disposed HEPA filters. In FIG. 24, a reference symbol " d" designates a vertical distance extending from the point where the measurement was carried out to the emitter points, a reference symbol "1" designates a horizontal distance extending from the point where the measurement was carried out to a vertical line passing through a central point of the ionizer, and the BACKGROUND data denote the positive and negative ion densities of the air flow when the ionizer was OFF. With the conventional AC type ionizers supplying positive ion rich air, the charged surface is not neutralized, rather it may remain positively charged at a potential on the order of several tens of volts to about +200 volts.